Kalocin

Kalocin, designated UJ-44759W or K-9, was created by Jensen Pharmaceuticals in the spring of 1965, and picked up by routine screening tests for new compounds. The first thing they noticed was that it inhibited growth, but it was soon found to inhibit and even cure cancer. It also killed viruses and bacteria; in fact it killed all of them. As Crichton writes:

"Somehow, the drug acted to destroy all organisms built on a unicellular structure, or less. It had no effect on organ systems - groups of cells organized into larger units. The drug was perfectly selective in this respect."

Knowledge of Kalocin was privately circulated and a further test was undertaken with healthy volunteers and cancer patients. All participants experienced massive diarrhea but, as Crichton puts it, "That seemed a small price to pay for a cancer cure." What happened next is one of the most fascinating excerpts from the book:

"On March 1, 1966, the forty men were taken off the drug. Within six hours, they were all dead.
It was what Jeremy Stone had predicted from the start. He had pointed out that mankind had, over centuries of exposure, developed a carefully regulated immunity to most organisms. On his skin, in the air, in his lungs, gut and even bloodstream were hundreds of different viruses and bacteria. They were potentially deadly, but man had adapted to them over the years, and only a few could still cause disease.
All this represented a carefully balanced state of affairs. If you introduced a new drug that killed all bacteria, you upset the balance and undid the evolutionary work of centuries. And you opened the way to superinfection, the problem of new organisms, bearing new diseases.
Stone was right: the forty volunteers each had died of obscure and horrible diseases no one had ever seen before. One man experienced swelling of his body, from head to foot, a hot, bloated swelling until he suffocated from pulmonary edema. Another man fell prey to an organism that ate away his stomach in a matter of hours. A third was hit by a virus that dissolved his brain to a jelly.
And so it went."